With the introduction of network servers and work stations that can utilize memory in the gigabyte range and can operate at speeds of 100 Mhz or higher, fast and high density memory modules are needed to reach these memory capacities and speeds. Present day computer systems typically include hundreds of discrete components mounted on printed circuit boards (PCBs) interconnected with wiring on the board. The PCBs may also include sockets and connectors for receiving additional components, component modules and multichip modules, and connectors to other PCBs.
Computer memory often consists of one or more memory modules which plug into connectors on main printed circuit boards in computers (motherboards). The PCB memory module connector sockets are interconnected by a common set of address, data and control lines. Generally, there are several memory module connectors and when the memory requirements increase, additional modules may be added onto the motherboards. However, as computer system speeds and memory requirements have continued to increase and more integrated devices are incorporated onto PCBs, traditional memory packaging schemes have become inadequate. A constant goal in designing integrated circuit (IC) modules is to pack more integrated circuitry into the same or less space. This may be accomplished by physically scaling down the electrical components, such as decreasing transistor size at the substrate level, thereby increasing transistor density on semiconductor chips. Another possibility has been to increase the number of integrated circuits on the PCB. With present PCB technology, ICs may be mounted on both surfaces (front and back) of the PCB using surface mount techniques. However, the PCBs generally cannot be increased in length or height due to space limitations imposed by available areas on motherboards and within computer housings thereby limiting the amount of PCB real estate available for additional memory ICs.
As the density requirements of modules increase, solutions are needed to meet these requirements. There are currently three solutions to meet the requirements. First, the individual PCB can be made larger to accommodate more memory chips, this includes folding the PCB in half using a flex circuit. However, increasing the number of chips on individual boards consequently increases the length of the traces between chips and other PCBs. The increase in the trace length has caused a deviation from standards which require certain lengths to be maintained in order to prevent skew among clock, address, and data signals. Other transmission line problems occur when these high speed signals are transmitted over traces that are too long. Such problems include reflections, cross-talk, and electromagnetic induction. Therefore, placement of memory ICs on PCBs is critical to design considerations when trying to increase memory capacity and density.
The second solution to increase memory density is to decrease semiconductor die size to fit more memory in the same semiconductor package. However, decreasing die size while increasing memory density leads to greater costs. The industry norm is a 64 Megabit die. There have been increases to a 128 Megabit and 256 Megabit die but with a corresponding increase in cost of approximately five to six times.
The third solution to increasing memory density is to stack semiconductor die in the same package. While this solution increases the memory density, heat dissipation becomes a problem. Each of the individual ICs become hot and the heat cannot be properly dissipated from the PCB. The increased heat causes the performance of the memory module to decrease and often fail. As a result, the memory modules cannot be run at full performance. Often clock speeds and data transfers have to be decreased to reduce heat generated by the modules. Moreover, heat generation problems limit the number of memory modules that can be populated on a PCB, degenerating performance. Therefore, the number of memory ICs that can be placed on any given PCB memory module is limited due to heat dissipation and other considerations.